The present invention relates to sheet-fed printing machines in general, and more particularly to a suction system for sheet guiding cylinders of a printing machine which is switchable for printing in one of two operational modes such as printing on one side of the sheet only, which is the first form mode, or printing on both sides of the sheet when the sheet is first printed on one side and thereafter is turned around its rear edge by means of the suction system and printed on its back side.
Suction systems of the type under consideration have been known. The suction system of this type disclosed in DD-PS No. 54,703 is arranged on the sheet guiding cylinder positioned between two printing cylinders. The sheet turning is carried out such that during the passing of the tangential point, that is the point of contact between two cylinders, the rear edge of the sheet to be turned is sucked by the suction system and the sheet is transported to the gripper system. The disadvantage of this otherwise satisfactory suction system resides in that the time for sucking the sheet to be turned over is very short. The suction system must be loaded with underpressure in a very short period of time and thus a precise control and high technical expenses connected therewith are required. Furthermore the increase in the machine output is also limited by short suction time.
The suction system disclosed in DD-PS No. 110,452 is already set on the leading edge of the sheet before the tangent point between the sheet guiding cylinder and the pre-arranged printing cylinder. The time available in this known design for suction the sheet is also too short because the sucker system must be moved over a trochoid path and, immediately after being set on the sheet trust, be controlled in the path from the printing cylinder to the sheet guiding cylinder. This type of control requires an extremely complex drive.
A pneumatic loading of the suction system has been disclosed in DD-PS No. 59102. The suckers in this publication are secured to a hollow swinging shaft on which air distributors are mounted. Air conduits lead from the air distributors to the hollow shaft of the sheet guiding cylinder. Each sucker is cyclically pneumatically loaded by a control shaft arranged adjacent to the sucker; this control shaft is mounted between the swinging shaft and the suction mouth.
The disadvantage of this air control device resides in that the underpressure which is necessary for the operation of the plurality of pneumatically loaded systems can be built up rather slow. Thereby register error can occur, which would negatively affect the printing output.